Many two stroke cycle engines utilize variable timing for the exhaust port opening. A common way to achieve this variable timing is by incorporating a slide valve that is adjustably mounted adjacent the exhaust port to vary the effective height of the exhaust port.
The tuning of the exhaust port provides added torque and power. At high power capacity at high speed, it is desirable for the exhaust port to open earlier compared to low speed and high torque conditions. The retardation of the opening of the exhaust port at lower idle speeds, helps maximize power, increases fuel efficiency and provides for cleaner burn with lower emissions.
The known slide gates, rotary valve members, and other restriction or barrier devices which delay opening of the exhaust port have been controlled by expensive mechanical devices or by use of the exhaust pressure. The exhaust pressure works against a diaphragm which can move the slide gate valve against a spring bias. A major detriment to using the exhaust pressure is that the exhaust gas contains many solid particulates formed in the combustion cycle that tend to plug the small passages used to the deliver the exhaust gas to the diaphragm.
Recently, externally scavenged two stroke cycle engines have been developed that incorporate a high speed centrifugal air compressor mechanically driven by the crankshaft. The air is driven from the compressor into the intake system that includes an intake air chest formed in the engine block that is separate from the crankcase. The intake system leads to the intake port of each cylinder.
The use of an external air compressor provides several advantages over piston scavenged two stroke cycle engines. The air compressor eliminates the need for the cylinder piston to function as an air pump and thereby eliminates the need for roller bearings and seals between cylinders about the crankshaft. The use of an external air compressor also allows use of the crankcase to house an oil pan and provide for separate oil lubrication. The need for mixing engine oil with the gasoline supply is thereby eliminated for such two stroke cycle engines. Oil is no longer burned as part of the fuel mixture which provides for a cleaner burn. Examples of such external compressors for this type of engine are disclosed in U.S. Pat. Nos. 5,058,546; 4,998,524; 5,425,345; and 5,423,304.
What is needed is a control system that uses the pressure within the intake system downstream of the external pump to control the timing of the opening of the exhaust valve in a two stroke cycle engine.